4-oxo-hexahydro-pyrazinoisoquinoline derivatives of the general Formula I ##STR1## wherein R.sup.2, R.sup.3 and R.sup.4 are each H or methyl and R.sup.5 and R.sup.6 are each H, methyl or methoxy, as well as their physiologically compatible acid addition salts, can be converted by acylation into the corresponding 2-acyl-4-oxo-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinolines with anthelmintic effectiveness and thus are important intermediate products. The compounds of Formula I are also themselves effective as anthelmintics.
In published German Patent Specification No. 23,31,713, corresponding to copending, commonly assigned U.S. Pat. application Ser. No. 481,792 filed June 24, 1974 and now U.S. Pat. No. 3,993,760, the contents of which are incorporated by reference herein, there is described a two-stage process for the preparation of 4-oxo-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinoline which starts from 2-benzoyl-4-oxo-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinoline. In the first reaction stage of this process, the benzoyl radical is split off by hydrolysis while the lactam ring is simultaneously split so that N-(1,2,3,4-tetrahydroisoquinolyl-1-methyl)-glycine results. The lactam ring is subsequently closed in the second reaction stage by heating. Yields are on the order of 70-80%.